The system is intended to be associated with an optical line transmitting a multiplex signal to enable some multiplexed optical signals to be dropped from it and other optical signals to be substituted for them using the wavelengths left available by the dropped signals.
Adding and dropping optical signals at particular wavelengths are operations which are indispensable to the communication of data in networks providing communication between geographically dispersed users. Systems therefore exist for this purpose and are used for injecting optical signals at predetermined wavelengths into multiplex signal transmission channels and selectively recovering data transmitted by the various channels. These drop-and-add systems are intended in particular for drop-and-add units providing selective service to users at selected locations in networks.
It is particularly important to obtain good performance in terms of optical signal transmission and in particular for the signals to suffer uniform losses during transmission, regardless of which wavelength from a particular range of available wavelengths is used to transmit data.
Also, signals which pass through a drop-and-add system on multiplex signal channels which are not dropped or added do not follow the same path as those which are added locally to the channels made available by the dropped signals. They are therefore not subject to the same insertion losses, which can lead to differences in the power levels of signals transmitted by different multiplex signal channels appearing at the output of a drop-and-add system. It is therefore important to prevent a system from causing penalizing insertion losses in some of the multiplexed optical signals that it transmits.
It has been found that these losses can be related to unequal attenuation at the various wavelengths used for the multiplex signal channels employed. Disturbances can also occur in the channels to which signals are added locally because of insufficient rejection of the dropped signals previously transmitted by those channels.
The invention therefore proposes a system for dropping/adding wavelength division multiplexed optical signals including an input and an output for an optical multiplex signal between which are added processors each of which is used for dropping an optical signal contained in one of xe2x80x9cNxe2x80x9d of the xe2x80x9cMxe2x80x9d channels of the incoming optical multiplex signal and adding an optical signal to the multiplex signal channel that is made available by the dropped signal.
According to a feature of the invention, each processor includes a drop optical filter and an add optical filter for an individual channel corresponding to a particular wavelength, an individual optical switch to which are connected a drop port of the drop filter, an individual add input, an individual drop output and an add port of the add filter, for dropping signals from and adding signals to a channel, and, to transmit the multiplex signal from one filter to the other for the purposes of dropping/adding signals, the drop filters of the circuits are connected in series from the input of the system and in order of decreasing channel wavelength in a drop subsystem, at the output of which the add filters are connected in series, and in order of decreasing channel wavelength in an add subsystem, a multiplex signal output port of each filter being connected to a multiplex signal input port of the next filter in the series, between the input and the output of the system, to which the input port of the first drop filter and the output port of the last add filter are respectively connected.
According to a feature of the invention, the difference between the insertion losses (IL_agg) for the multiplexed optical signals passing through the system which cannot be added or dropped and the insertion losses (IL_AD) for the multiplexed optical signals which are transmitted at the xe2x80x9cNxe2x80x9d channel wavelengths which can be dropped, as given by the expression
ILxe2x80x94ADxe2x88x92IL_agg=IL_drop+IL_sw+IL_add+(N+1)xc3x97IL_thru
is a minimum for the number N of processing circuits provided, IL_drop corresponding to the drop losses for a drop filter, IL_sw corresponding to the losses on passing through an optical switch, IL_add corresponding to the insertion losses for an add filter, and IL_thru corresponding to the through losses between multiplex signal input ports and multiplex signal output ports of a drop or add filter.
According to a feature of the invention, a rejection filter is incorporated in each circuit downstream of the multiplex signal output port of the drop filter of the circuit to increase therein the rejection level vis-à-vis the signal which is dropped there, the rejection filter being either directly between that output port and the multiplex signal input port of the drop or add filter immediately following it for alternate drop filters from the first of the series or between the input port of the add filter of the circuit and the output port of the add or drop filter immediately preceding it to which input port of that rejection filter is then connected.
According to a feature of one embodiment of the invention, each circuit includes two rejection filters to increase the rejection level of the circuit vis-à-vis the optical signal which is dropped there, one of the two filters being connected downstream of the multiplex signal output port of the drop filter of the circuit in the drop subsystem and the other upstream of the input port of the add filter of the circuit in the add subsystem.
According to a feature of the invention, a fixed level-adjusting attenuator is added between the output of the drop subsystem through which the multiplex signal from the last drop filter passes and the input port of the first filter of the add subsystem by which the multiplex signal is received.
According to a feature of the invention, a processor includes a drop filter whose drop port is connected to a first optical splitter to enable simultaneous transmission of the signal which it drops to the add filter of the circuit for re-insertion in the multiplex signal leaving the system and to an individual drop output of the system which is allocated to the circuit.
According to another feature, a processor includes a second optical splitter connected to an individual signal add input of the system to enable simultaneous transmission of the signal received from that input to the add filter of the circuit and to the individual drop output which is allocated to the circuit, such transmission being effected via at least one switch of the circuit.
According to another feature, the system includes optical splitters with two asymmetric outputs.
According to a final feature, the add, drop and rejection filters are of the MIF, IFBG, MZ-IFBG type or the fiber coupler type obtained by fusion-drawing and including a centrally located Bragg grating.